It's a decentralized social media protocol. More rich and versatile than RSS.
To the original commenter: yes. This project is using ActivityPub kind of like RSS. Since it's ActivityPub and not merely RSS, the experience when using e.g. Mastodon as your reader, however, is "like RSS, worse". On the other hand, since it's ActivityPub, it also supports all the ways to track followups/replies/threads both from the original poster and people who are not the original poster, a way to dynamically track/display who else is subscribed, and it advertises a channel for you to send notifications to if you yourself wanted to reply or follow. This is its main value proposition—mostly for people who prioritize the gimmick of modern-ish social features over simplicity and economical setup/hosting.
Now if the blog uses ActivityPub and federates with Mastodon instances, then it's not a blog anymore, right?
Or similarly, if Lemmy is supposed to be like Reddit, then it should not federate with Mastodon (everything that federates with Mastodon kind of becomes a Twitter alternative, doesn't it?).
Probably I haven't understood the idea yet.
If your blog uses ActivityPub, it is still a blog. People could just subscribe to it to receive your posts in their home feed, reply to them, or re-share them in a way your blog could process (or choose not to).